Chemicals of interest have been tested for mutagenicity using a number of Salmonella tester strains. The chemicals studied this year, or being studied, were diethylstilbestrol (DES), sodium bisulfite (SB), sodium azide, halothane, a series of nitrosamines, and pyrene. DES has yielded negative results in all mutation assay systems used. SB, an antioxidant, has been shown to be mutagenic at low pH, and the mutagenicity was bacterial strain-specific. Sodium azide, a known mutagen, and widely used as a positive control in the Salmonella assays, can form a volatile mutagen. This study was done in order to show if production of the volatile mutagen can interfere with the results on plates not containing azide but in the same incubator as plates with azide. The benzo(a)pyrene analog, pyrene, which up to now has given inconsistent mutagenicity data, was tested with a newly constructed Salmonella strain TA97 as well as some of the others.